Coaxial-to-waveguide power divider and directional coupler



Dec. 17, 1963 w. J. WILSQN 3,114,888

COAXIAL-TO-WAVEGUIDE POWER D IVIDER AND DIRECTIONAL COUPLER Filed June 30, 1961 82 SOURCE William J. Wilson IN VEN TOR gwam AT TORNE Y United States Patent Delaware Filed June 30, 1961, Ser. No. 121,051 Claims. (Cl. 333) This invention relates to an improved coupler for interchanging energy between a waveguide and a transmission line. The coupler, which comprises an unshielded transmission line inner conductor passing transversely through the waveguide, has non-directional characteristics when the inner conductor is perpendicular to the longitudinal axis of the guide. Coupling is directional when the inner conductor is oblique to the longitudinal axis.

In the high frequency art, it is often desirable to efficiently transfer electromagnetic energy between a waveguide and a transmission line, such as coaxial or strip line. For example, feeding an array of strip line antenna elements from a waveguide may call for a power dividing coupler, wherein part of the energy in the waveguide is coupled to each of two strip lines. The balance of the energy continues along the waveguide to energize other elements of the antenna array. The relative phase or amplitude of the energy transferred to the two strip lines may be critical, for example, to obtain a desired antenna radiation pattern.

In other applications, directional coupling between the waveguide and the transmission line is desired, such as in monitoring the power reflected back toward a transmitter from an antenna.

It is a principal object of my invention to provide an improved four-port waveguide transmission line coupler.

A further object of my invention is to provide a coupler of the above type having non-directional characteristics.

Another object of my invention is to provide an improved waveguide transmission line coupler wherein two transmission line branches are energized with a frequencyindependent phase difference between them.

A further object of my invention is to provide a coupler of the above type having directional characteristics.

A still further object of my invention is to provide couplers of the above type that are readily fabricated.

Other objects of the invention will in part be obvious and will in part appear hereinafter,

The invention accordingly comprises the features of construction, combinations of elements and arrangements of parts which will be exemplified in the constructions hereinafter set forth and the scope of the invention will be indicated in the claims.

In general, a power-dividing coupler embodying my invention incorporates a transmission line inner conductor that passes unshielded through a waveguide parallel to the electric field of the dominant waveguide transmission mode. The transmission line extends from both sides of the waveguide, and its outer conductor is connected to the waveguide walls. With this construction, the power in the waveguide can be coupled equally to both transmission line branches when the transmission line inner conductor is perpendicular to the waveguide axis. The portions of the waveguide power coupled to the respective branches depend on the relative impedances they present to the waveguide. More specifically, unequal power is coupled to the two branches when they present different impedances to the waveguide.

When the inner conductor is disposed obliquely to the longitudinal axis of the waveguide, coupling is directional, the directivity varying with the angle between the inner conductor and the waveguide.

The phase difference between the power coupled to the two transmission line branches is degrees. Both the power dividing coupler and the directional coupler are reciprocal, i.e., energy in the transmission line is coupled to the waveguide according to the same conditions governing the coupling of waveguide energy to the transmission me.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompartying drawings in which:

FIGURE 1 is an end view, partly broken away, of a power-dividing coupler embodying my invention,

FIGURE 2 is a longitudinal section taken along line 2-2 of the coupler of FIGURE 1,

FIGURE 3 is a longitudinal vertical section of a directional coupler made according to my invention, and

FIGURE 4 is a fragmentary, enlarged longitudinal vertical section of a simplified representation of the coupler of FIGURE 3.

As shown in FIGURES l and 2, a power-dividing coupler embodying my invention has a transmission line, generally indicated at it that passes perpendicularly through a waveguide indicated at 12. The waveguide is preferably rectangular, having a pair of Wide walls 14 and 16 and a pair of narrow walls 18 and 20.

Transmission line 10 may be a strip transmission line having a flat ribbon-like inner conductor 22 symmetrically disposed within an outer conductor system comprising a pair of parallel ground plane conductors 24 and 26. An insulator 28 fills the space between the inner conductor 22 and the outer conductors '24 and 26. The outer conductors are removed from the portion of the transmission line ill within the waveguide 12, and, thus, the inner conductor 22 passes unshielded through the waveguide. The portion of transmission line extending from the waveguide wall 14 is termed a transmission line branch 30. Similarly a branch 32 of transmission line 10 extends below the waveguide from wall 14'. Flanges, such as a flange 34, are secured to each end of the waveguide 12 to facilitate connection of the coupler in a high frequency circuitv As described below, energy propagating to the right (FIGURE 2) in waveguide 12 is coupled to the strip transmission line branches 30 and 32. The portion of the energy in the waveguide coupled to each branch depends on the impedances the branches present to the waveguide relative to the waveguide impedance.

Still referring to FIGURES 1 and 2, the outer conductor system of the transmission line branches 30 and 32 is firmly connected to the waveguide :12 so as to have substantially the same potential as the wide walls 14 and 16, respectively.

More specifically, the transmission line 10 is connected to the wide wall 14 with a clamp, generally indicated at 36, having a fixed jaw 38 secured to the wall 14 as by silver soldering and a movable jaw 4i bolted to jaw 33. The jaws 33 and 40 are preferably formed with recesses 38a and 45a that receive the outer conductors 24 and 26. Machine screws 42-42 clamp the jaws 38 and 40 against the outer conductors adjacent the point where the outer conductors terminate. Branch 32 of the transmission line is connected to wall 16 in a similar manner, with a clamp 44 comprising a fixed jaw 46 and a movable jaw 48.

As pointed out above, inner conductor 22 passes transversely through the waveguide 12, normal to the walls 14 and 16 and parallel to the walls 18 and 20. Accordingly, it is parallel to the electric field of the dominant TE waveguide transmission mode. Preferably it passes through the waveguide midway between the walls '18 and 20, to maximize coupling and minimize excitation of higher order transmission modes in the waveguide and the transmission line.

Coupling between the waveguide and the transmission line branches 30 and 32 may be explained by considering energy propagating in a TE waveguide mode to the right (FIGURE 2) in the waveguide 12. The time varying electric field between the wide waveguide walls 14 and 16 induces a potential gradient along the inner conductor 22 which excites currents in the conductor. The magnetic field in the waveguide induces currents in the inner conductor that are in phase with the currents resulting from the electric field.

The resultant current thus induced has the same direction throughout the section of the conductor 22 within the waveguide 12. Accordingly, since the potentials of the walls 14 and 16 areof opposite polarity, the power coupled to the branch 30 is 180 degrees out of phase with the power coupled to branch 32. Power not coupled to the transmission line branches 30 and 32 continues to the right in the waveguide '12 or is reflected back to the left.

Since it is a reciprocal device, the power-dividing coupler of FIGURES 1 and 2 operates with equal efiiciency to couple energy from transmission line to waveguide 12, .the coupled power dividing between the portions of the waveguide extending to the right and to the left from the line.

Referring now to FIGURE 3, a directional coupler embodying the features of my invention is similar to the power dividing coupler of FIGURES 1 and 2, except that the transmission line inner conductor intersects the Waveguide obliquely to its longitudinal axis. More specifically, the directional coupler includes a waveguide 52 formed with a pair of wide walls 54 and 56 and a pair of narrow walls including a wall 58. Flanges 6t and 62 may be secured to each end of the waveguide 52. A strip transmission line 50, intersecting the Waveguide at an angle 0, has an inner conductor 64, ground plane outer conductors 66 and 68 and insulators 7070 The transmission line is secured to walls 54 and 56 with clamps indicated at 72 and 74, which may be similar to the clamps 36 and 44 described above with reference to FIG- URES 1 and 2.

Inner conductor 64 is unshielded within the waveguide 52. As will be described more fully below, part of the high frequency energy delivered from a source 75 to waveguide 52 at a port 76 is directionally coupled to transmission line branches indicated at 78 and 80. The balance of the energy continues along the waveguide 52 to a port 82.

More particularly, referring to FIGURE 4, the section of inner conductor 64 within waveguide 52 may be considered as comprising a succession of infinitesimal segments 92a, 92b. Energy propagating along the Waveguide in a TE transmission mode is coupled to each segment 92a, 92b, in a manner analogous to the coupling described above for the coupler of FIGURES 1 and 2.

Consider two segments 92a and 92b, 92a being spaced above wall 56 the same distance segment 92b is spaced above the waveguide center line 96. Energy coupled to segment 92b by a wave propagating to the right in the waveguide is delayed in phase from energy coupled to segment 92a by an amount proportional to the electrical distance along the waveguide between the segments. Signals propagating along the inner conductor 64 between the segments 92a and 92b undergo a phase delay proportional to the electrical distance between them along conductor 64.

Current arriving at port 80 from segment 92b is delayed in phase with respect to current arriving from segment 92a by an amount equal to the sum of the two phase delays described above. Currents from the two segments will cancel each other when their relative phase difference is equivalent to an odd multiple of a half-wavelength. This condition can be achieved at a given frequency by adjusting the angle 0.

On the other hand, the currents will combine to develop a maximum signal at port 86 when their relative phase difierence is equivalent to a wavelength.

The entire length of the inner conductor '64 within the waveguide is composed of pairs of segments similar to segments 92a and 92b. Thus, the current at port 80, which is the sum of the individual currents from each pair of segments, can be continuously varied from a minimum to a maximum value merely by adjusting the angle between the waveguide and the transmission line.

The current delivered to port 78 also varies as the angle is adjusted. However, it generally varies inversely from the current at port 8% so that directional coupling is obtained between the waveguide 52 and transmission line 50.

From the above, it is seen that the direotivity is a function of the angle 0, at which the transmission line 50' intersects the axis of the waveguide 96. Furthermore, it can be shown that the coupler of FIGURE 3 is a reciprocal device, in that energy delivered to the transmission line branch 78 is coupled selectively toward the waveguide ports 76 and 82.

Thus, I have described a waveguide-transmission line coupler that is particularly adapted for power-dividing applications. It incorporates a transmission line inner conductor that passes unshielded within the waveguide parallel to the electric field therein. The transmission line outer conductor is connected to the walls of the waveguide adjacent to the entry of the inner conductor into the waveguide. The portion of the power in the waveguide coupled to the transmission line branches extending from each side of the guide is readily varied by adjusting the impedance of each branch relative to the characteristic impedance of the waveguide.

I have also described a coupler of similar construction having directional characteristics. In this case the transmission line inner conductor is disposed obliquely to the electric field in the waveguide. The directivity is adjustable by varying the angle between the inner conductor and the waveguide electric field. Strip and coaxial transmission lines are particularly well adapted for use in the couplers.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiic-iently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. A directional coupler comprising, in combination, a waveguide having a transverse axis and extending along a longitudinal axis, a transmission line having an inner conductor and an outer conductor, said inner conductor passing unshielded through said waveguide perpendicularly to said transverse axis and obliquely to said longitudinal axis, said outer conductor being electrically coupled to walls of said waveguide adjacent to the points where said inner conductor enters said waveguide.

2. A directional coupler between waveguide and transmission line comprising, in combination, a waveguide having a pair of narrow sides and a pair of wide sides, a transmission line comprising an inner conductor symmetrically disposed within an outer conductor system, said transmission line passing through said waveguide with said inner conductor extending between said wide sides obliquely thereto and parallel to said narrow sides, said outer conductor system being disposed outside said waveguide and terminating at said wide sides.

3. The combination defined in claim 2 in which said transmission line is a strip transmission line and in which said outer conductor system comprises a pair of parallel ground plane conductors.

4. A waveguide-transmission line directional coupler comprising, in combination, a waveguide having a pair of narrow walls and a pair of wide walls, a transmission line having first and second ports and comprising an inner conductor symmetrically disposed within an outer conductor system, said transmission line passing through said waveguide with said inner conductor extending unshielded be tween said wide walls parallel to said narrow walls, said outer conductor system being connected to said wide walls of said waveguide adjacent to the points where said inner conductor passes therethrough, said inner conductor being disposed at an angle with respect to said wide walls so that signals in said waveguide are coupled to said first port substantially in phase with each other, whereby said signals are directionally coupled to said first port.

5. A waveguide-transmission line directional coupler comprising, in combination, a waveguide having a pair of narrow walls and a pair of wide walls, a longitudinal waveguide axis midway between said wide walls, a flange secured to said waveguide at one end thereof for connecting a source thereto, a transmission line having first and second ports and comprising an inner conductor symmetrically disposed within an outer conductor system, said transmission line passing through said waveguide with said inner conductor passing unshielded between said wide Walls parallel to said narrow walls, said outer conductor system being connected to said wide waveguide walls adjacent to the points where said inner conductor passes therethrough, said inner conductor being oriented within said iwaveguide to form an acute angle with said wide walls, the angle between said line and said waveguide axis being such that at the frequency of said source a first signal induced in said inner conductor at any given distance therealong from a first wide wall and a second signal induced in said inner conductor at the same distance therealong on the other side of said axis from said first wall destructively interfere with each other at said second port and augment each other at said first port.

Barker Oct. 9, 1956 Seidel Dec. 6, 1960 

1. A DIRECTIONAL COUPLER COMPRISING, IN COMBINATION, A WAVEGUIDE HAVING A TRANSVERSE AXIS AND EXTENDING ALONG A LONGITUDINAL AXIS, A TRANSMISSION LINE HAVING AN INNER CONDUCTOR AND AN OUTER CONDUCTOR, SAID INNER CONDUCTOR PASSING UNSHIELDED THROUGH SAID WAVEGUIDE PERPENDICULARLY TO SAID TRANSVERSE AXIS AND OBLIQUELY TO SAID LONGITUDINAL AXIS, SAID OUTER CONDUCTOR BEING ELECTRICALLY COUPLED TO WALLS OF SAID WAVEGUIDE ADJACENT TO THE POINTS WHERE SAID INNER CONDUCTOR ENTERS SAID WAVEGUIDE. 